Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.
Zenmap - Zenmap is the official cross-platform GUI for the Nmap Security Scanner.
Wireless Network Watcher - scans your wireless network and displays the list of all computers and devices that are currently connected to your network.
Advanced Port Scanner - Advanced Port Scanner is a fast and free software for port scanning.